Many people add whey protein isolate to their coffee, an easy way to get your protein without having to eat solid food (eggs, meat, dairy etc).
Pouring hot coffee over protein powder can resulted in a lot of gelled clumps that did not mix in, even with tons of stirring, and the protein got stuck to the bottom of the mug. There are many ways to mix whey protein with coffee, but this is the simplest and best way to mix it, without the need of blender.
Before put your coffee (or any other beverage) into your cup, put first the whey protein, then add some cool water and mix them well (Whey protein mixes well with cool water). After mixing them, just add your coffee in the cup. Now drink your protein-powered coffee.